Apoptosis is a form of programmed cell death which occurs through the activation of cell-intrinsic suicide machinery. The biochemical machinery responsible for apoptosis is expressed in most, if not all, cells. Apoptosis is primarily a physiologic process necessary to remove individual cells that are no longer needed or that function abnormally. Apoptosis is a regulated event dependent upon active metabolism and protein synthesis by the dying cell.
The morphological and biochemical characteristics of cells dying by apoptosis differ markedly from those of cells dying by necrosis. During apoptosis, cells decrease in size and round up. The nuclear chromatin undergoes condensation and fragmentation. Cell death is preceded by DNA fragmentation. The DNA of apoptotic cells is nonrandomly degraded by endogenous calcium and magnesium-dependent endonuclease(s) inhibited by zinc ions. This enzyme(s) gives fragments of approx. 200 base pairs (bp) or multiples of 200 bp by cutting the linker DNA running between nucleosomes. Thus DNA appears to be one of the most important targets of the process that leads to cell suicide. The apoptotic cell then breaks apart into many plasma membrane-bound vesicles called “apoptotic bodies,” which contain fragments of condensed chromatin and morphologically intact organelles such as mitochondria. Apoptotic cells and bodies are rapidly phagocytosed, thereby protecting surrounding tissues from injury. The rapid and efficient clearance of apoptotic cells makes apoptosis extremely difficult to detect in tissue sections.
In contrast, necrosis is associated with rapid metabolic collapse that leads to cell swelling, early loss of plasma membrane integrity, and ultimate cell rupture. Cytosolic contents leach from the necrotic cell causing injury and inflammation to surrounding tissue.
In contrast to the cell death caused by cell injury, apoptosis is an active process of gene-directed, cellular self-destruction and that it serves a biologically meaningful function. (Kerr, J. F. R and J. Searle. J. Pathol. 107:41, 1971). Apoptosis plays a key role in the human body from the early stages of embryonic development through to the inevitable decline associated with old age. (Wyllie, A. H. Int. Rev. Cytol. 68:251, 1980). The normal function of the immune, gastrointestinal and hematopoietic system relies on the normal function of apoptosis. When the normal function of apoptosis goes awry, the cause or the result can be one of a number of diseases, including: cancer, viral infections, auto-immune disease/allergies, neurodegeneration or cardiovascular diseases. Because of the versatility of apoptosis involved in human diseases, apoptosis is becoming a prominent buzzword in the pharmaceutical research field.
The idea of modulating apoptosis as a means of treating and/or preventing cancer is a relatively new idea (Cope, F. O and Wille, J. Apoptosis: The Molecular Basis of Cell Death. Cold Spring Harbor Laboratory Press, p. 61, 1991). Apoptosis modulation is a potential mechanism for controlling the growth of tumor cells without the side effects of many current cancer treatment regimes. In addition to cancer, recent studies show that multiple cytotoxic stimuli well known to cause necrosis can lead to apoptosis instead when cells are exposed to the same noxious agents at lower concentrations.
Malignant tumors (cancers) are the second leading cause of death in the United States, after heart disease (Boring et al., CA Cancel J. Clin., 43:7 (1993)).
Cancer is characterized by the increase in the number of abnormal, or neoplastic, cells derived from a normal tissue which proliferate to form a tumor mass, the invasion of adjacent tissues by these neoplastic tumor cells, and the generation of malignant cells which eventually spread via the blood or lymphatic system to regional lymph nodes and to distant sites (metastasis). In a cancerous state a cell proliferates under conditions in which the normal cells would not grow. Cancer manifests itself in a wide variety of forms, characterized by different degrees of invasiveness and aggressiveness.
Despite recent advances in cancer therapy, there is a great need for new therapeutic agents capable of inhibiting neoplastic cell growth. Accordingly, an objective of the present invention is methods and compositions capable of inhibiting the growth of neoplastic cells, such as cancer cells, by inducing apoptosis and necrosis.